1. Field of the Invention
The present invention is directed to an antenna arrangement for a nuclear magnetic resonance apparatus for exciting nuclear spins in an examination subject and for receiving the resulting magnetic resonance signals from the subject.
2. Description of the Prior Art
As is known, magnetic radiant fields and radio-frequency fields for the excitation of nuclear spins in a subject are required in addition to a strong basic magnetic field in a nuclear magnetic resonance imaging apparatus. A Helmholtz coil arrangement, with the examination lying in the interior thereof, is generally employed in combination with superconducting magnets, which generate the basic magnetic field. The direction of the basic magnetic field thus lies along the longitudinal direction of the patient to be examined. A radio-frequency field is generated for deflecting the nuclear spins, this field having magnetic field components in directions extending perpendicularly relative to the direction of the basic magnetic field. An antenna structure as disclosed in U.S. Pat. No. 4,506,224, for example, can be employed for this purpose. In this structure, rods extend in the longitudinal direction of the magnet, i.e., in the direction of the basic magnetic field.
Nuclear magnetic tomography systems are also known wherein the basic magnetic field is generated by an electromagnet or a permanent magnet. These types of magnets usually have a C-shape or an H-shape, or are constructed with four columns. The basic magnetic field is generated between two pole plates, and thus extends perpendicularly relative to the longitudinal direction of the patient. The open examination space of such magnet structures has a number of advantages over the closed, tubular examination space which is standard in the case of a superconducting magnet. The examination volume remains relatively accessible during the examination, so that examinations requiring the intervention of a physician or attending personnel can be implemented. Additionally, the risk that the patient being examined will suffer from claustrophobic conditions is remote in such open magnet systems.
For magnets having pole plates, there is a desire to keep the pole plate spacing as small as possible, so that the weight of the magnet remains low, and a better homogeneity of the basic magnetic field can be achieved. Any components which are built-in between the pole plates, for example the radio-frequency antennas, must therefore be kept as flat as possible. Such antennas must also be shaped so that accessibility to the examination volume for interventional examinations is not impeded.
European Application 0 359 374 discloses an antenna arrangement wherein each pole plate has a sub-antenna with relatively large-area conductors, as well as with a return conductor, allocated thereto. The large-area conductors interact with the return conductor as a transmission line that is tuned to the radio-frequency. The large-area conductors at the two pole plates are respectively supplied with current flowing in opposite directions. A disadvantageous of this known arrangement is that disturbances in the radio-frequency distribution are caused by the return conductor.